Oil sands extraction processes primarily use hot water mixed with oil sands ore to produce a slurry from which is removed a froth fraction containing bitumen. The bitumen froth, which contains bitumen, water and fine mineral solids, is further processed by adding a diluent solvent to facilitate separation of the bitumen from the other components.
In froth treatment operations, the bitumen froth is mixed with diluent and the diluted froth is supplied to separation vessels to separate an overflow diluted bitumen stream from an underflow solvent diluted tailings stream.
Froth treatment operations thus produce by-products including solvent diluted tailings. The cost and environmental impact preclude directly discharging solvent diluted tailings to tailings ponds. The diluted tailings are thus treated in a tailings solvent recovery unit.
Various tailings solvent recovery units have been proposed and each has its own set of drawbacks and challenges. Many possible recovery schemes are disclosed in the literature. In one process, froth treatment tailings from the froth treatment plant are introduced into a flash vessel with internal shed decks maintained at sub-atmospheric pressures. Steam is introduced below the internals and the major portion of the diluent vaporizes together with water. The flashed vapours are removed and cooled to condense diluent and water which separate by gravity settling. Non-condensed vent gases are withdrawn from the condenser to maintain the sub-atmospheric pressure. The flashed solvent depleted tailings are pumped from the flash vessel to tailings disposal.
Some challenges encountered by known tailings solvent recovery processes result in lower solvent recovery levels than would be desirable. For some processes, the lower recovery is attributable to premature flashing at the feed inlet inducing feed to bypass the shed decks and negating any addition of steam below the shed decks. Other processes which operate the flash vessel at near atmospheric pressures which may permit feed distribution over the shed decks and may increase the steam addition to maintain vessel temperature to about 100° C. can increase naphtha diluent recovery.
Another diluent recovery process investigation flashes feed to a flash temperature such that the enthalpy of vaporized flash components matches enthalpy released from the flash liquid and the flash temperature governs vapour pressures of vaporizing components. Given the relative volatility of diluent hydrocarbons, there may be an expected direct relationship between feed temperature, flash temperature and diluent recovery. However, the investigation identified increased feed temperatures for the same feed flow did not proportionately translate to increased diluent recovery due to increased vaporization of water. Stable operation for the flash column in terms of flash temperature and pressure was found marginally below the boiling point of water for the operating pressure and with small increases in feed enthalpy resulting in upsets as the water essentially boils.
Process upsets affect the flash column in at least two ways. Firstly, boiling on shed decks results in damage to the extent that frequently the shed decks fail structurally. Secondly, the vapour velocity in the column increases by an order of magnitude exceeding design guidelines, such a set out in “Design Two-Phase Separators within the Right Limits” W. Svrcek, et al. Chemical Engineering Progress October 1993, to limit entraining solids and bitumen into the overhead system.
In the overhead of the tailings solvent flash column, bitumen acts a binder for the solids to adhere on surfaces in the overhead system. The adherence of solids to components of the overhead system restricts vapour flow to the downstream equipments unit operations such as condensers and separators. The adherence of solids on condenser heat transfer surfaces reduce cooling and condensing of vapours which increases the non-condensed gases to be vented. Directionally, both effects of solids adhering on surfaces in the overhead system increase column pressure which reduces feed flashing resulting in actual diluent recoveries. The contribution of increased steam to improve diluent recoveries due the reduced partial pressure created by the superheated steam can often be largely offset by the increased water vapour reporting an overhead system restricted by the adherence of solids. Over the operating cycle, the deposit of solids causes column performance to deteriorate which can only be regained by shutting down the column and associated systems for repair and cleaning.
As mentioned above, some known processes use flash vessels with internal shed decks to provide a large surface area to facilitate flashing or stripping of diluent from froth treatment tailings. The flash conditions are near the boiling point of water and both feed flow and feed temperature variations occur. For flash columns depending on internal shed decks there are a number of challenges and drawbacks. For instance, high feed flows increase liquid loading on column internals which directionally increases the time required for diluent to separate. In addition, low feed flows can cause short circuiting in the column when feed does not adequately cover internals and permits depositions of froth treatment tailings mineral and bitumen/asphaltenes on internal surfaces. The depositions provide sites to increase corrosion of the shed decks. Furthermore, high feed temperatures which boil the water on the internals with resultant vibrations and shock can lead to structural failure of the shed decks, which can be seen observing shed deck pieces in the bottom of the column. In general, these challenges reflect feed conditions as supplied from the froth treatment plant to the tailings solvent treatment unit.
Other known or proposed units have columns substantially free of internals with an agitated liquid pool with residence time to allow residual solvent to evaporate while limiting foaming. The column generally has an inlet device that finely disperses the feed. Other known or proposed technologies describe specific manifold assemblies for injecting feed in a tailings solvent recovery vessel that may have small openings through which the feed is injected.
In other known units, such as described in US patent application published under No. 2010/0282642 (Kan et al.), a column is provided such that it is substantially free of internals using nozzles with diameters of about 0.5 mm with a precise pressure drop range producing substantially solvent depleted hydrocarbon drops which reduce in size to a precise range after a fall to the bottom of the vessel over a precise time interval. The inlet feed nozzles may be orientated up or down and the column may also have steam nozzles to inject steam counter-currently with respect to the falling droplets. This proposed system may have a number of challenges and drawbacks. For instance, feed to tailings solvent recovery columns can contain some “tram” materials. This tram material may include prehistoric wood and coal which are contained in the bitumen froth and during froth solvent diluent addition the tram material follows the froth treatment tailings stream. Due to screening or grinding limitations, such tram material may be in the range of 10 to 15 mm and thus can cause plugging, clogging or altered flow through nozzles of 0.5 mm. In addition, due to variable feed conditions, controlling the unit for the precise pressure drops, droplet size distribution and evolution over precise drop times may be difficult. Furthermore, feed injection systems and demisting systems are themselves internal to the flash vessel and thus bitumen/asphaltenes can adhere also to their surfaces leading to maintenance and cleaning issues.
There is thus a need for a technology that overcomes at least some of the challenges and drawbacks of what is known in the field.